Active solid-state devices (e.g. – transistors – solid-state diode – Thin active physical layer which is – Heterojunction
Reexamination Certificate
1998-07-28
2001-01-09
Meier, Stephen D. (Department: 2822)
Active solid-state devices (e.g., transistors, solid-state diode
Thin active physical layer which is
Heterojunction
C257S014000, C257S441000
Reexamination Certificate
active
06172379
ABSTRACT:
FIELD AND BACKGROUND OF THE INVENTION
The present invention relates to quantum well infrared photodetectors (QWIP) and, more particularly, to a simple way to optimize the geometry of a grating of a QWIP with respect to detection of light of a certain frequency, and the QWIP so designed.
QWIPs are devices for detecting medium and long wavelength infrared light. These devices rely on quantum wells, typically multiple quantum wells, to provide effective bandgaps that are narrower than can be achieved easily in homogeneous semiconductors. The theory and design of QWIPs is reviewed by B. F. Levine in “Quantum-well infrared photodetectors”,
Journal of Applied Physics
vol. 74 no. 8 (Oct. 15, 1993), pp. R1-R81.
FIG. 1
is a schematic cross-section of a typical QWIP
10
. QWIP
10
consists of parallel layers of a low-band-gap semiconductor
14
embedded in a relatively higher-band-gap semiconductor
12
. For example, semiconductor
14
may be GaAs and semiconductor
12
may be Al
x
Ga
1−x
As. Ellipsis
16
indicates that there typically are many more layers than are shown in FIG.
1
. In fact, a typical QWIP includes on the order of 50 periods of alternating layers
12
and
14
. Layers
14
are quantum wells. This structure of alternating layers
12
and
14
is formed on a GaAs contact layer
13
above a GaAs substrate
15
and is capped by a GaAs contact layer
13
′.
Many variations of the illustrative example of
FIG. 1
exist. For example:
(a) Layers
12
,
13
,
13
′ and
15
may be Si, and layers
14
may be Si
x
Ge
1−x
. (b) Layers
12
,
13
,
13
′ and
15
may be InP, and layers
14
may be InGaAsP or InGaAs. (c) Layers
13
,
13
′,
14
and
15
may be GaAs and layers
12
may be GaInP. (d) On a GaAs substrate, alternating barrier layers of AlGaAs and multilayer quantum wells; each quantum well consists of a sandwich of a central InGaAs layer between two GaAs layers; thin tunneling barrier layers of AlAs intervene between the quantum wells and the AlGaAs layers.
Other variations may be found in Levine's review article.
Because these semiconductors have indices of refraction, with respect to the propagation of infrared light, that are significantly greater than 1, infrared light incident from below on front surface
18
of QWIP
10
at almost any angle of incidence is refracted to propagate almost perpendicular to quantum wells
14
. This makes the electric field vector of the light almost parallel to quantum wells
14
. Unfortunately, it is only the component of the electric field perpendicular to quantum wells
14
that interacts with quantum wells
14
. A common way to overcome this problem is to provide a two-dimensional grating
22
, parallel to quantum wells
14
, on back surface
20
of QWIP
10
to scatter the light, thereby causing the light to propagate within QWIP
10
in directions oblique and parallel to quantum wells
14
as well as perpendicular to quantum wells
14
.
The geometry of grating
22
is defined by three parameters: pitch p, cavity width w, and cavity depth d. Pitch p define the lateral periodicities of grating
22
. To enhance the performance of QWIP
10
with respect to infrared light of a frequency v, i.e., a free-space wavelength &lgr;=c/&ngr; (where c is the speed of light in a vacuum) by promoting constructive interference of light scattered parallel to grating
22
, p is set equal to the wavelength of the light inside QWIP
10
, &lgr;
, where n is the index of refraction of semiconductor
13
′ with respect to light of frequency &ngr;. Note that the infrared light for which the performance of QWIP
10
is optimized is defined herein in terms of frequency rather than in terms of wavelength to avoid confusion between the free-space wavelength of the light and the wavelength of the light within QWIP
10
.
Two methods are known for selecting cavity depth d. The first is to use a simple rule of thumb, as taught by Chi et al. in U.S. Pat. No. 5,075,749. Light reflected from back surface
20
at an angle smaller than the critical angle of total internal reflection within QWIP
10
escapes from front surface
18
. To suppress this, d is selected to promote destructive interference of light scattered perpendicular to grating
22
. Specifically, to enhance the performance of QWIP
10
with respect to infrared light of frequency &ngr;, d is set equal to c/4n&ngr;, one-quarter of the wavelength of the light inside QWIP
10
.
The second is to solve Maxwell's equations for the electromagnetic field inside QWIP
10
for a suite of values of d and to select the value of d that maximizes the quantum efficiency of QWIP
10
. For example, J. Y. Andersson and L. Lundqvist, in “Grating-coupled quantum-well infrared detectors: theory and performance”,
Journal of Applied Physics
vol. 71 no. 7 (Apr. 1, 1992) pp. 3600-3610, used the modal expansion method to calculate quantum efficiencies of a model QWIP
10
at various values of w and d to determine optimal values of w and d.
The regular geometry of grating
22
is not the only possible geometry. Levine et al., in U.S. Pat. No. 5,506,419, which is incorporated by reference for all purposes as if fully set forth herein, teach a QWIP grating with a pseudo-random geometry. In one variant of the pseudo-random geometry, the lateral dimensions of the grating cavities varies pseudo-randomly, while the depths of the cavities can have one of several values. These depths are selected as multiples of the quarter wavelength taught by Chi et al. in U.S. Pat. No. 5,075,749. It should be noted that the vector computation of Andersson and Lundqvist can be performed only for a grating such as grating
22
that has a regularly periodic geometry, and not for the pseudo-random geometries of Levine et al., U.S. Pat. No. 5,075,749.
SUMMARY OF THE INVENTION
The present invention falls in complexity between the rule of thumb of Chi et al., U.S. Pat. No. 5,075,749 and the full-blown vector calculation of Andersson and Lundqvist. Surprisingly, it has been found that a simple scalar calculation of quantum efficiency gives a QWIP design superior in performance to that taught by Chi et al. in U.S. Pat. No. 5,075,749, and nevertheless is applicable to nonperiodic grating geometries such as the pseudo-random geometry of Levine et al., U.S. Pat. No. 5,506,419.
Therefore, according to the present invention there is provided a method for selecting at least one depth of a grating of a QWIP to enhance detection of light of a certain frequency, including the steps of: (a) forming a scalar expression for an intensity of the light within the QWIP, as a function of the at least one depth; (b) estimating a quantum efficiency of the QWIP, based on the scalar expression, at a plurality of values of the at least one depth; and (c) selecting one of the plurality of the values that maximizes the quantum efficiency.
Furthermore, according to the present invention there is provided a QWIP for detecting light of a certain frequency, the light having a certain wavelength when propagating within the QWIP, including: (a) at least one planar quantum well; and (b) a grating, parallel to the at least one planar quantum well, having a depth greater than an odd multiple of one-quarter of the wavelength and less than the odd multiple of about three-tenths of the wavelength.
Furthermore, according to the present invention there is provided a QWIP for detecting light of a certain frequency, including: (a) at least one planar quantum well; and (b) a grating, parallel to the at least one planar quantum well, and having at least one depth selected by estimating a quantum efficiency of the QWIP based on a scalar expression for an intensity of the light within the QWIP, the scalar expression being a function of the at least one depth.
Furthermore, according to the present invention there is provided a method for selecting at least one depth of a grating of a QWIP to enhance detection of light in a plurality of frequency bands, including the steps of: (a) selecting a representative frequency from among the frequency
Friedman Mark M.
Meier Stephen D.
Sagi-Nahor Ltd.
LandOfFree
Method for optimizing QWIP grating depth does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Method for optimizing QWIP grating depth, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Method for optimizing QWIP grating depth will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2544062